1. Field of the Invention
The present invention is directed to a power divider for waveforms having strong higher harmonics, such as square waves, and particularly to such a power divider that imparts a phase difference of 180.degree. between branches for radio communication.
2. Description of Related Art
Many RF (radio-frequency) and microwave communication systems, especially those using mobile telephones such as cellular and PCS (personal communication system) telephones, rely on the transmission of square waves. The square wave is rich in harmonic content, specifically odd-order harmonics. More specifically, a square wave of fundamental frequency f has harmonics of order 3f, 5f, 7f, . . . , or in other words .xi.f, where .xi. is an odd integer. The amplitude of each harmonic of order .xi., normalized to the amplitude of the fundamental, goes as 1/.xi., as shown in FIG. 1.
Accurate amplification and delivery of a square wave to the individual stages require division of this RF waveform. For generation of large output power levels, the driving waveform is divided into a minimum of two waveforms. Each waveform is fed into an amplifier stage, amplified, and then recombined with the other waveforms.
There are currently several techniques for dividing the waveform. Current techniques to divide a waveform for use by multiple amplifiers include the Wilkinson hybrid, radial wave power hybrid, multiport power divider using circular-sector-shaped planar components, and many other types of microwave hybrids. However, because of design constraints, namely, the extremely wide bandwidth needed for a waveform with significant higher-order harmonics, the current power dividers are incapable of dividing a harmonically rich waveform.
Each of the current dividers has drawbacks that make it unusable in a harmonically rich environment such as that of the High Efficiency Microwave Power Amplifier (HEMPA). Each topology will be shown to exhibit characteristics that make it unsuitable.
The Wilkinson hybrid is a popular method for increasing the power output of amplifier sections. This method of dividing is inherently easy to design and implement. There are some drawbacks, however. First, the number of devices combined in this type of topology is restricted to powers of two (i.e., 2.sup.N where N is a positive integer). Second, the combining efficiency decreases rapidly as the number of outputs increases because of the strong dependence on the losses within the divider. Also, the size of the divider increases drastically as the number of outputs increases.
For waveforms free of harmonics, the Wilkinson hybrid coupler is an attractive choice for RF engineers. However, since square waves are harmonically rich, the Wilkinson hybrid coupler is unusable with such waves because of certain restrictions imposed by its design.
The first restriction deals with the limited bandwidth of the hybrid coupler. The usable bandwidth of most Wilkinson hybrids is 1 to 1.5 octaves. The waveform that is inherent in the HEMPA is more than four octaves in bandwidth. This in itself makes the Wilkinson hybrid unusable.
The second problem associated with the Wilkinson hybrid deals with the main problem of the Wilkinson topology and the nonplanarity of the circuit due to the presence of the floating node connecting the isolation resistor to the outputs.
With these two major problems associated with the Wilkinson hybrid, the use of this topology in the HEMPA is precluded.
The Radial wave power hybrid incorporates a radially periodic internal structure. This structure uses techniques similar to those used for distributed element filter design. This method offers a superior approach to high-order, high-power combining. The general guideline for this type of divider deals with an impedance matching problem whereby an equivalent source impedance of Z.sub.0 /N must be matched to the common port impedance. This is usually done using a lumped element filter network, thus imparting a frequency dependence to the network. Therefore, this type of topology is also unusable for applications using square waves.
The final type of power divider uses circular shaped planar components. The circular-sector topology has one major drawback in that the usable bandwidth is very narrow (i.e., approximately 10% of the center frequency). This major constraint makes the circular sector topology useless.
This difficulty in dividing and combining square waves has limited the efficiency of power generation in cellular telephones. The present state of the art in amplification at microwave frequencies has been precluded from incorporating high-efficiency techniques because of the above-noted constraints inherent to the design.
Another problem with known dividers is that they have lengths in increments of .lambda./4, where .lambda. is the wavelength of the waveform. In this case, the fundamental component, having a wavelength of .lambda., travels one-quarter of its own wavelength. However, the third-order (.xi.=3) harmonic, having a wavelength of .lambda./3, travels over three-quarters of its own wavelength. The fundamental and third-order components are thus made to be 180.degree. out of phase. The fifth-order (.xi.=5) harmnonic, having a wavelength of .lambda./5, travels over five-quarters of its own wavelength, or a complete cycle plus one-quarter of its own wavelength, and thus is maintained in phase with the fundamental. Accordingly, the harmonics are alternately in phase and 180.degree. out of phase with the fundamental. As a consequence, the waveform is distorted beyond usability.
Cellular-telephone manufacturers spend millions of dollars per year on research and development to improve cellular-telephone technology. One area of research deals with increasing the battery life and operating time, which are currently limited by the efficiency of power amplification.